A SynRM utilizing solely the reluctance principle to create torque does not have any conductors in the rotor. Therefore, in comparison with induction machines and machines with field excitation windings, the rotor of a SynRM has significantly lower losses and thereby a lower operation temperature. Since it has been conventionally considered that a SynRM does not need cooling of the rotor, a conventional SynRM does not comprise air circulating means within the machine housing. The stator of a conventional SynRM is cooled by providing an external fan in a non-driving end (N-side) of the machine housing. The air flow from the fan is directed to follow the envelope surface of the machine housing. Because the fan is at one end of the machine and the cooling effect is higher closer to the fan, there is a significant temperature difference between the drive end (D-side) and the N-side of the machine.
U.S. Pat. No. 5,831,367 discloses a rotor for a synchronous reluctance machine. It has been mentioned on column 6, lines 37 to 40 that air circulation can be achieved by providing squirrel cage rings with fins. Cooling in the rotor according to U.S. Pat. No. 5,831,367 is required since the motor utilizes both reluctance principle and induction principle to create torque. It is the induced current in the squirrel cage conductor bars that mostly heats up the rotor, and the teaching of the required cooling can therefore not be directly applied to a machine not comprising any conductors on the rotor. Furthermore, U.S. Pat. No. 5,831,367 does not describe details about how the air is circulated within the machine housing.
Since a conventional SynRM does not comprise air circulating means within the machine housing, the interior of the machine housing does not comprise passage ways for circulating air. A typical rotor of a SynRM comprises axial passages through which air can flow, but since a typical stator does not have a return flow path through the stator or radial outside of the stator, providing the outer radius of the rotor with fins according to U.S. Pat. No. 5,831,367 would not cause air to effectively circulate within the machine housing. Modifications to the stator or the motor housing of a conventional SynRM would therefore be required in order to cause the air to circulate.
US 2006/0222528 discloses a synchronous reluctance machine wherein flux barriers are used as cooling channels. Gaseous refrigerant is forced to flow through the flux barriers in a single direction.
US 2007/0024130 discloses an asynchronous machine wherein cooling air is flowing in two opposite directions within the rotor through channels provided particularly for cooling purpose.
US 2007/0024129 discloses an asynchronous machine, more precisely an induction machine, wherein the rotor and the stator are provided with cooling channels extending in axial direction of the machine. Air is flowing in two opposite directions in different cooling channels before it is exhausted to the ambient of the machine housing. The channels in the rotor and the stator are provided particularly for cooling purpose, and the air is caused to flow in two directions in order to prevent non-uniform cooling between the opposite axial ends of the machine.